Focal length
and f-ratio

The focal length determines how wide an
angle the telescope can view with a giveneyepiece or size of a CCD detector or photographic plate. The f-ratio(or focal ratio, or f-number)
of a telescope is the ratio between the focal length and the aperture (i.e.,
diameter) of the objective. Thus, for a given aperture
(light-gathering power), low f-ratios indicate wide fields of
view. Wide-field telescopes (such as astrographs) are used to track satellites and asteroids, for cosmic-ray research, and for surveys
of the sky. It is more difficult to reduce optical aberrations in telescopes with
low f-ratio than in telescopes with larger f-ratio.